The cover image of my updated works On Consciousness: Science & Subjectivity shows five cortico-thalamic projection systems, each one operating in similar ways, and each one projecting to different regions of cortex. Steriade (2006) has emphasized that the thalamus is not a true center of signaling, but rather that it is the cortex that resonates with closely related thalamic nuclei. Thalamic nuclei essentially bounce back their cortical input signals to cortex, showing again that resonance is the working style of the great “cortico-thalamic” system.
Bidirectional signaling is the standard working mode for the waking cortex, both the outer cortical mantle of the neocortex and the inner double-horned hippocampus and the sensory projection areas for taste and smell.
Steriade (2006) has emphasized that the thalamus is not a true center of signaling, but rather that it is the cortex that resonates with closely related thalamic nuclei. Thalamic nuclei essentially bounce back their cortical input signals to the cortex, showing again that resonance is the working style of the great “cortico-thalamic” system.
The image only shows five cortico-thalamic projection systems, each one operating in similar ways, and each one projecting to different regions of cortex. Additional white matter pathways run between the right and left hemispheres via the corpus callosum, front to back in bidirectional pathways from V1 (for example) to the prefrontal cortex, body map connections between the sensory and motor homunculi, and corticospinal pathways running upward and downward. There are also major local and intermediate pathways, most famously between Broca’s and Wernicke’s area for speaking and speech perception. By current estimates this system has about 80 billion nerve cells, and quadrillions of synaptic connections.
This represents perhaps the most biologically expensive organ in the body because it requires an uninterrupted supply of oxygen and glucose, rapid toxin disposal, and vulnerability to injury, infection and attack. This high biological cost suggests that the cerebral cortex has corresponding Darwinian fitness benefits.
The basic unit of the C-T is therefore not the single neuron, nor the traditional one-directional sensory pathway. It is rather “a unit of adaptive resonance,” which can be thought of as an artificial neural network with at least two layers. Bidirectionality makes the C-T core different from the cerebellum and basal ganglia, which do not support conscious contents directly. Damage to the cerebellum does not directly impair conscious contents, though it can devastate fine motor control. The cerebellum has parallel modules and pathways comparable to a computer server farm, while the C-T complex is parallel-interactive, so that any array of neurons can signal any other. While the cerebellum is more “cognitive” than previously thought, it does not directly enable conscious contents.
Most C-T activity is endogenous: “the cortex talks mostly to itself.” With a few exceptions thalamic nuclei do not communicate with each other directly, but are driven by cortical regions. Steriade (2006) concluded,
“The cerebral cortex and thalamus constitute a unified oscillatory machine displaying different spontaneous rhythms that are dependent on the behavioral state of vigilance.” (Italics added.)
To read more, purchase your copy of ON CONSCIOUSNESS: Science & Subjectivity – Updated Works on Global Workspace Theory.
Global Workspace Theory (GWT) began with this question: “How does a serial, integrated and very limited stream of consciousness emerge from a nervous system that is mostly unconscious, distributed, parallel and of enormous capacity?”
GWT is a widely used framework for the role of conscious and unconscious experiences in the functioning of the brain, as Baars first suggested in 1983.
A set of explicit assumptions that can be tested, as many of them have been. These updated works by Bernie Baars, the recipient of the 2019 Hermann von Helmholtz Life Contribution Award by International Neural Network Society form a coherent effort to organize a large and growing body of scientific evidence about conscious brains.
